Using FFT and IFFT (matlab code) to transform frequency domain to time domain

I am using HP8753ES VNA. I got the frequency response from 100k-6GHz and also the time domain response for a coaxial cable. When I used the FFT to transform the time domain signal to frequency domain, why the result is different from what I directly got from the VNA? similarily, the IFFT also didn't work well.